# operator==,!=,<,<=,>,>=,<=>(std::forward_list)

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< cpp‎ | container‎ | forward list

Defined in header `<forward_list>` |  |
template< class T, class Alloc > bool operator==( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (1) | (since C++11)
template< class T, class Alloc > bool operator!=( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (2) | (since C++11) (until C++20)
template< class T, class Alloc > bool operator<( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (3) | (since C++11) (until C++20)
template< class T, class Alloc > bool operator<=( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (4) | (since C++11) (until C++20)
template< class T, class Alloc > bool operator>( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (5) | (since C++11) (until C++20)
template< class T, class Alloc > bool operator>=( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (6) | (since C++11) (until C++20)
template< class T, class Alloc > synth-three-way-result<T> operator<=>( const std::forward_list<T, Alloc>& lhs, const std::forward_list<T, Alloc>& rhs ); | (7) | (since C++20)

Compares the contents of two `forward_list`s.

1,2) Checks if the contents of lhs and rhs are equal, that is, they have the same number of elements and each element in lhs compares equal with the element in rhs at the same position.
3-6) Compares the contents of lhs and rhs lexicographically. The comparison is performed by a function equivalent to std::lexicographical_compare.
7) Compares the contents of lhs and rhs lexicographically. The comparison is performed as if by calling std::lexicographical_compare_three_way(lhs.begin(), lhs.end(),
`` rhs.begin(), rhs.end(),`` `synth-three-way`).
The return type is the return type of `synth-three-way` (i.e., `synth-three-way-result`﻿<T>).
If none of the following conditions is satisfied, the behavior is undefined:

- `T` models `three_way_comparable`.
- `<` is defined for values of type (possibly const-qualified) `T`, and `<` is a total ordering relationship.

The `<`, `<=`, `>`, `>=`, and `!=` operators are synthesized from operator<=> and operator== respectively. | (since C++20)

### Parameters

lhs, rhs | - | `forward_list`s whose contents to compare
- `T` must meet the requirements of EqualityComparable in order to use overloads (1,2).
- `T` must meet the requirements of LessThanComparable in order to use overloads (3-6). The ordering relation must establish total order.

### Return value

1) true if the contents of the `forward_list`s are equal, false otherwise.
2) true if the contents of the `forward_list`s are not equal, false otherwise.
3) true if the contents of the lhs are lexicographically less than the contents of rhs, false otherwise.
4) true if the contents of the lhs are lexicographically less than or equal to the contents of rhs, false otherwise.
5) true if the contents of the lhs are lexicographically greater than the contents of rhs, false otherwise.
6) true if the contents of the lhs are lexicographically greater than or equal to the contents of rhs, false otherwise.
7) The relative order of the first pair of non-equivalent elements in lhs and rhs if there are such elements, lhs.size() <=> rhs.size() otherwise.

### Complexity

Linear in the size of the `forward_list`.

### Notes

The relational operators are defined in terms of the element type's operator<. | (until C++20)
The relational operators are defined in terms of `synth-three-way`, which uses operator<=> if possible, or operator< otherwise. Notably, if the element does not itself provide operator<=>, but is implicitly convertible to a three-way comparable type, that conversion will be used instead of operator<. | (since C++20)

### Example

Run this code

    #include <cassert>
    #include <compare>
    #include <forward_list>

    int main()
    {
        const std::forward_list
            a{1, 2, 3},
            b{1, 2, 3},
            c{7, 8, 9, 10};

        assert
        (""
            "Compare equal containers:" &&
            (a != b) == false &&
            (a == b) == true &&
            (a < b) == false &&
            (a <= b) == true &&
            (a > b) == false &&
            (a >= b) == true &&
            (a <=> b) != std::weak_ordering::less &&
            (a <=> b) != std::weak_ordering::greater &&
            (a <=> b) == std::weak_ordering::equivalent &&
            (a <=> b) >= 0 &&
            (a <=> b) <= 0 &&
            (a <=> b) == 0 &&

            "Compare non equal containers:" &&
            (a != c) == true &&
            (a == c) == false &&
            (a < c) == true &&
            (a <= c) == true &&
            (a > c) == false &&
            (a >= c) == false &&
            (a <=> c) == std::weak_ordering::less &&
            (a <=> c) != std::weak_ordering::equivalent &&
            (a <=> c) != std::weak_ordering::greater &&
            (a <=> c) < 0 &&
            (a <=> c) != 0 &&
            (a <=> c) <= 0 &&
        "");
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3431 | C++20 | operator<=> did not require `T` to model `three_way_comparable` | requires

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